1. Technical Field of the Invention
The present invention relates to a battery pack constructed with a plurality of battery modules that are respectively constituted by unitizing a plurality of cells.
2. Description of Related Art
It has been the practice to constitute a battery pack, in order to achieve a necessary output power, when employing rechargeable batteries. In a battery pack, generally, a plurality of battery modules, each being constituted by connecting a plurality of cells in series and bonding them into a unitized body, are arranged in parallel and mutually connected in series.
FIG. 11 shows a conventional battery module made by connecting a plurality of cells and coupling them together as one. In this battery module, a plurality of cells 41 (41a to 41j) are arranged side by side, with the long lateral walls of their cell cases opposite each other. End plates 52 are arranged against the outside of the cells 41a and 41j at both ends, and the group of cells and the two end plates 52 are bound together with binding bands 53 so as to couple the cells together as one piece.
For the cells 41, as shown in FIG. 12, an electrode plate group 47, comprising positive electrode plates and negative electrode plates layered with intervening separators, thereby constituting elements for electromotive force, is accommodated in a cell case 42 together with a liquid electrolyte, and the open end of the cell case 42 is closed with a lid 46 provided with a safety vent 45. From the upper end at one side of the positive electrode plates forming the electrode plate group 47, leads 49 extend upward and are connected to a positive electrode terminal 43 above them, and similarly, from the upper end of the other side of the negative electrode plates, leads 49 extend upward and are connected to an negative electrode terminal 44 above them. The positive electrode terminal 43 and the negative electrode terminal 44 are attached to the lid 46.
The positive electrode terminals 43 and negative electrode terminals 44 of coupled neighboring cells 41 are connected by connection plates 51, thereby connecting all cells 41 in series. When the cell cases 42 are coupled, ribs 48, which protrude vertically from the long lateral walls of the cell cases 42, are abutted against each other, forming coolant passages running in the vertical direction along the long lateral walls of the cell cases 42 in the space between ribs 48, so that the cells 41a to 41j are cooled by flowing air through these coolant passages.
In the battery pack employing such battery module as described above, because the plurality of cells 41 (41a-41j) are tightly bound together, if the internal pressure rises in some of the cells 41 due to generation of gas induced by, for example, overcharging, whereupon the cell cases 42 of these cells 41 expand, then the other cells adjacent the expanding cells become depressed in order to accommodate the expansion. Thus the internal pressures in each of the cells 41 are maintained substantially uniform in each of the battery modules. Therefore, as far as one battery module is concerned, even if there is an imbalance in the internal pressure between various cells 41 whereupon the output characteristics and the life of some cells deteriorate, the output characteristics and the service life of the battery module as a whole are relatively unaffected.
However, when a plurality of the battery modules as shown in FIG. 11 are bound together to constitute a battery pack, the cells 41 of one battery module will not be bound in tight contact with the cells 41 of adjacent battery module, and there will be variations in the internal pressure between the cells 41 of different battery modules. Therefore, it can happen that the internal pressure in the cells 41 of some battery modules become higher than that in the cells 41 of the other battery modules, and when this battery pack is taken as a whole, the output characteristics and service life of the battery pack may deteriorate because of degradation in the output characteristics or service life of some of the battery modules.
Moreover, the battery module shown in FIG. 11 is constituted by unitizing a plurality of cells, and in order to constitute a battery pack, a plurality of such battery modules are coupled together. Accordingly, the battery pack becomes relatively bulky with a complex structure and heavy weight. There is thus the problem that the battery pack as a whole has relatively low percentages of output by volume and by weight.
The higher the power output of each cell is, the higher the heat generation in the cell becomes. When the temperature in the cell rises, the internal pressure rises accordingly due to expansion of electrode plates or generation of gas, which adversely affects the output characteristics and service life of the cell. The battery pack therefore needs to meet the requirements for high cooling performance. However, with the construction of the prior art battery pack constituted with battery modules shown in FIG. 11, it is hardly possible to further improve the cooling performance, because the cell cases 42 of each cell 41 are made of synthetic resin having low thermal conductivity, and the cells are cooled by passing air through the coolant passages between the cells.